C++ Functions

Function

• Need to include function prototype before function can be used
  • void DoMagic(Magic name);
• Passing parameters
  • C++ is pass by value
  • Use pointer to pass by pointer (dah)
    
void TestPassByPointer(){
int num = 10;
Incr10(&num);
return;
}
// Pass by a copy of pointer
// so same memory address
int Incr10(int* num){
*num += 10;
return *num;
}
  • Pass by reference
    
void TestPassByRef(){
int num = 10;
int& rnum = num;
Incr_10(rnum);
return;
}
// Pass by reference
// Changes the original variable directly
int Incr_10(int& num){
num += 10;
return num;
}
  • Mandate that parameter not be changed
    • int increase10(const int& num);
  • Initialize parameters
    • int myfxn(long arg1 = 10, long arg2 = 20);
• Return values
  • Return pointer
    • Never return the address of a local variable from a function (memory is gone!)
    • Instead, return a pointer to free store and don’t forget to delete (memory leak!)
      
double* treble(double data)
{
// Allocate memory in the heap
double* result = new double(0.0);
//Check out of memory
if (!result){
cout << "Out of memory..." << endl;
exit(1);
}
*result = 3.0*data;
return result;
}
void TestReturnPointer()
{
double num = 10.0;
double* ptr = 0;
ptr = treble(num);
cout << "three times " << num << " is " << *ptr << endl;
delete ptr; // Don't forget to delete!
return;
}
  • Return reference

Pointer to function

• Declare
  • return_type (*pointer_name) (list_of_parameter_types);
  • double (*pfun) (char*, int);
• Example
  
void TestFunctionPointer()
{
// Declar a function pointer
long (*fptr)(long, long);
// Assign fxn pointer to product
fptr = product;
cout << fptr(3, 5) << endl;
// Assign fxn pointer to sum
fptr = sum;
cout << fptr(3, 5) << endl;
}
long product(long a, long b)
{
return a*b;
}
long sum(long a, long b)
{
return a+b;
}
• Function pointer can be passed as parameter
  • double sumarray(double [] array, int len, double (*pfun)(double));

Function Template

• Example
  
// Define template
template T max(T x[], int len)
{
T max = x[0];
for (int i = 0; i < len; i++)
{
if (max < x[i])
max = x[i];
}
return max;
}
void TestFunctionTemplate()
{
int small[] = {1, 25, 35, 15};
int lensmall = sizeof small/sizeof small[0];double medium[] = {25, 250, 2500, 5, 450};
int lenmedium = sizeof medium/sizeof medium[0];cout << "Max of small: " << max(small, lensmall) << endl;
cout << "Max of medium: " << max(medium, lenmedium) << endl;
}